In receivers of communication terminal equipment, such as radio receivers for cellular handsets, a frequency conversion is performed to convert a radio frequency (RF) signal into a baseband (BB) or intermediate frequency (IF) signal. Such frequency conversion is typically accomplished by mixing an input signal, which is input from a signal source, with a local oscillator (LO) signal. This basic principle is applied in different types of receiver architectures such as e.g. direct conversion and mixer-first (LNA-less) receiver architectures.
FIG. 1 shows a schematic diagram of an exemplary topology of a direct-conversion receiver architecture.
As is evident from FIG. 1, a received radio signal is preselected by a pre-select (band-pass) filter, and the thus preselected radio signal is amplified in a low-noise amplifier (LNA) before being down-converted. The down-conversion may be performed with two down-conversion mixers (MIX) controlled by a local oscillator (LO) signal, which is divided into an in-phase LO signal and a quadrature LO signal with 90° phase shift, to prevent signal sidebands from aliasing on one another. In each receive path, prior to analog-to-digital conversion by an analog-to-digital converter (ADC), the signal is low-pass filtered by a low-pass filter (LP) and amplified by an amplifier (AMP) such that the signal level for the ADC is at a sufficient level.
FIG. 2 shows a schematic diagram of an exemplary topology of a mixer-first (LNA-less) receiver architecture.
As is evident from FIG. 2, there is no active amplifier, such as the LNA in FIG. 1, between a pre-select (band-pass) filter and a further processing of the thus preselected radio signal. Otherwise, the topology of FIG. 2 corresponds to that of FIG. 1, and the processing in each receive path is basically similar to that described above in connection with FIG. 1.
In both receiver architectures described above, there is a problem in that the LO signal could leak through the mixers (which could be assigned to a frequency converter), i.e. that LO leakage signal components could appear at the input. If so, corresponding LO leakage signal components would result in DC components in an output signal of the frequency converter which are not based on static DC offsets or second-order linearity (IIP2) effects but on self-mixing effects. These can however not be distinguished from DC components caused by other effects, such as temperature-dependent static DC offset or even-order nonlinearity effects, for example.
While this problem is also present in a direct-conversion receiver architecture, it is even more relevant in a mixer-first (LNA-less) receiver architecture. This is due to the lack of an amplifier such as a LNA at the receiver input, which leads to the lack of reverse isolation for LO leakage towards the antenna, and also the lack of an active amplification stage to increase the level of the wanted signal at the mixer.
One specific problem caused by LO leakage to a receiver input is that the LO signal contains phase noise that appears as additive noise at the receiver input and deteriorates the noise figure of the receiver. FIG. 3 shows a graph illustrating a receiver noise figure over LO leakage in a mixer-first (LNA-less) receiver architecture.
As is evident from FIG. 3, the deterioration of a receiver noise figure from leaked LO phase noise becomes worse with decreasing receiver bandwidth. For the LTE bandwidth modes 1.4, 3, 5, 10, 15 and 20 MHz, the receiver noise figure at every LO leakage is larger in this sequence. For example, the curve for the LTE 5 MHz mode, i.e. the third curve from the top, shows that LO leakage of −68 dBm at the input of a RFIC, which has a noise figure of 2.5 dB in the absence of LO leakage, will increase that noise figure to 5 dB.
Accordingly, narrow-band modes appear to be more sensitive to LO leakage (or phase noise), as most of the phase noise spectrum is confined in a bandwidth even smaller than 1.4 MHz. In other words, the absolute phase noise contributed to the LTE 1.4 MHz mode is essentially the same as for the LTE 20 MHz mode, regardless of the difference in receiver bandwidth, but the relative phase noise becomes larger with decreasing receiver bandwidth.
While calibration of a frequency converter (or its mixers) could reduce LO leakage and thus the resulting DC components, a receiver cannot directly sense its own LO leakage, as it converts to 0 Hz at baseband and appears in combination with DC offsets from other sources. Therefore, no such accurate calibration is feasible, but only “symptoms” of the LO leakage, i.e. resulting effects, can be observed, and a calibration can be based on adding the observed “symptoms” with opposite sign, resulting in an inaccurate calibration.
Accordingly, techniques of leakage calibration for a frequency converter are required for reducing local oscillator leakage of the frequency converter.